Muhammad Ali was too ill to meet the royal couple in person Credit: Reuters

Muhammad Ali welcomed the Prince of Wales and Duchess of Cornwall to his home city of Louisville, describing Charles as "the greatest".

The sport star's words came in a letter rather than in person as he was not well enough to join leading figures from the city at a cultural greeting for the royal couple, who were in the city on the final day of their US tour.

(My wife) Lonnie and I welcome you to our hometown of Louisville, Kentucky, USA. We are honoured that you have come to explore, share and learn about new sustainable initiatives that are so close to your heart.

Louisville prides itself on being a compassionate city and we are confident that you will leave feeling a sense of our southern hospitality, caring for the environment, and yes - our love for college basketball.

As you travel back to your homeland, we hope you know how much this city respects and admires the many contributions you have made in the world.